The present invention relates to a method of making light-polarizing particles of improved particle size distribution for use in liquid suspensions and light valves, films, and set suspensions.
Light valves have been known for over sixty years for modulation of light. As used herein, a light valve may be described as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains a light-modulating element, which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed and encapsulated.
The liquid suspension (sometimes herein called a liquid light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
Light valves have been proposed for many purposes including e.g., alpha-numeric displays, television displays, windows, sun roofs, sun visors, mirrors, eyeglasses and the like to control the amount of light passing therethrough. Light valves of the type described herein are also known as xe2x80x9csuspended particle devicesxe2x80x9d or SPDsxe2x80x9d.
For many applications, it is preferable for the activatable material to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects e.g., bulging associated with a high column of liquid suspension can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that in a plastic film the particles are generally present only within very small droplets, and hence do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A xe2x80x9clight valve filmxe2x80x9d as used herein is thus a film having droplets of a liquid suspension of particles distributed in the film.
A type of light valve film made by phase separation from a homogeneous solution is disclosed in U.S. Pat. No. 5,409,734. Light valve films made by cross-linking emulsions are disclosed in U.S. Pat. Nos. 5,463,491 and 5,463,492 assigned to the assignee of the present invention. All of those patents and other patents and other sources cited herein are incorporated herein by reference thereto.
For use in set suspensions such as light-polarizing sheets, sometimes called xe2x80x9csheet polarizersxe2x80x9d, which can be cut up and formed into polarized sunglass lenses or used as filters, light-polarizing particles can be dispersed or distributed throughout a sheet of suitable film-forming material, such as cellulose acetate or polyvinyl alcohol or the like. Methods of making set suspensions for use in sheet polarizers are well known in the prior art. In these set suspensions, however, the particles are immovable. See e.g., U.S. Pat. Nos. 2,178,996 and 2,041,138.
1. Liquid Suspending Media And Stabilizers
The liquid light valve suspension may be any liquid light valve suspension known in the art and may be formulated according to known techniques. The term xe2x80x9cliquid light valve suspensionxe2x80x9d as used herein means a xe2x80x9cliquid suspending mediumxe2x80x9d in which a plurality of small particles are dispersed. The xe2x80x9cliquid suspending mediumxe2x80x9d comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
The liquid light valve suspension of the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art are useful herein, such as but not limited to the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general one or both of the liquid suspending medium or the polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer when employed, can be a single type of solid polymer that bonds to the surface of the particles but also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose, which in effect, provides a plain surface coating for the particles and one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
2. Particles
As is known, inorganic and organic particles may be used in a light valve suspension. However, the present invention relates to an improved method of preparing particles that are polyhalides (sometimes referred to in the prior art as perhalides) of organic compounds, such as alkaloid acid salts and the like. The polyhalide particles of the present invention may be light-polarizing, such as halogen-containing light-polarizing materials, e.g., polyhalides of alkaloid acid salts. (The term xe2x80x9calkaloidxe2x80x9d is used herein to mean an organic nitrogenous base, as defined in Hackh""s Chemical Dictionary, Fourth Edition, McGraw-Hill Book Company, New York, 1969). As is known, if a polyhalide of an alkaloid acid salt is prepared, the alkaloid moiety may be a quinine alkaloid, as defined in Hackh""s Chemical Dictionary, supra. U.S. Pat. Nos. 2,178,996 and 2,289,712 refer in detail to the use of polyhalides of quinine alkaloid acid salts. The particles may be light-absorbing or light-reflecting. Also, the particles may be particles of a hydrogenated polyhalide of a quinine alkaloid acid salt, such as dihydrocinchonidine sulfate polyiodide, as described in U.S. Pat. No. 4,131,334.
More recently, improved polyhalide particles having advantageous features for use in light valves have been proposed in U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463. These xe2x80x9cpolyhalide particlesxe2x80x9d are formed by reacting organic compounds, usually containing nitrogen, with elemental iodine and a hydrohalide acid or an ammonium alkali metal halide or alkaline earth metal halide. Such organic compounds are referred to herein as a xe2x80x9cPrecursorxe2x80x9d.
Prior art polyhalide particles are also discussed in detail in xe2x80x9cThe Optical Properties and Structure of Polyiodidesxe2x80x9d by D. A. Godina and G. P. Faerman published in The Journal of General Chemistry, U.S.S.R. Vol. 20, pp. 1005-1016 (1950). Herapathite, for example, is quinine bisulfate polyiodide, and its formula is given under the heading xe2x80x9cquinine iodosulfatexe2x80x9d as 4C20H24N2O2.3H2SO4.2HI.I4.6H2O in The Merck Index, 10th Ed. (Merck and Co., Inc., Rahway, N.J.). In polyiodide compounds, the iodide anion is thought to form chains and the compounds are strong light polarizers. See U.S. Pat. No. 4,877,313 and Teitelbaum et al. JACS 100 (1978), pp. 3215-3217. The term xe2x80x9cpolyhalidexe2x80x9d is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another halide anion.
As is known, polyhalide particles that are useful for light valves are preferably of colloidal size, that is the particles will have a largest dimension averaging about 1 micron or less. It is preferred that most polyhalide particles have their largest dimension less than one-half of the wavelength of blue light i.e., 2000 Angstroms or less to keep light scatter extremely low.
The present invention provides a method of preparing polyhalide particles that are especially well suited for use as the particles of a liquid light valve suspension, which comprises reacting a xe2x80x9cPrecursorxe2x80x9d of a specified particle size with elemental iodine and a hydrohalide acid or an ammonium, alkali metal or alkaline earth metal halide. The Precursor may be any of the compounds previously used to form organic polyhalide particles by reaction with elemental iodine and a hydrohalide acid or an ammonium, alkali metal or alkaline earth metal halide. For example, the Precursor may be a quinine alkaloid acid salt (U.S. Pat. Nos. 2,178,996 and 2,289,712), a hydrogenated alkaloid acid salt (U.S. Pat. No. 4,131,334) or an organic compound containing one or more groups that chelate hydrogen, ammonium or metal ions (U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463), all of such U.S. patents being incorporated herein by reference thereto. The Precursor can be any color but generally is comprised of small white or off-white colored crystals (sometimes referred to herein as xe2x80x9cparticlesxe2x80x9d).
We have surprisingly found that if the average size and/or median size of the Precursor is less than 1 micron, preferably less than 0.75 micron, the quality of the polyhalide particles made therefrom is substantially improved. Comminution (size reduction) of the Precursor particles to provide the desired particle size may be accomplished by any means that reduces their size, provided that the process does not cause the comminuted particles to cluster or clump up, which could offset the advantages of comminution and might actually cause the effective size of the particles to increase. For example, the Precursor particles can be pulverized or ground with a mortar and pestle or with a ball mill or any other convenient means, either dry or wet with a liquid, or with another solid inert substance present to aid pulverization. Alternatively, the Precursor particles can be caused to collide with one another by being subjected to rapidly moving gas streams, for example, by being blasted with a supersonic stream or streams of air.
As used herein, if Precursor particles or crystals are said to have been comminuted or reduced in size, what is meant is that their average size and/or their median size has been reduced. The xe2x80x9csizexe2x80x9d of a particle as used herein means and refers to the particle""s largest dimension.
The present invention is illustrated in terms of its preferred embodiments in the following Examples.
A typical modern prior art type of polyhalide particle (crystal) is pyrazine-2,5-dicarboxylic acid dihydrate calcium iodide polyiodide. A procedure for making such crystals and a liquid suspension thereof for use in a light valve is disclosed in Example 1.